This invention relates to a resonant converter and more particularly to a resonant converter which is suitable to assure the stabilized operation of switching devices used therein.
Self-excited oscillation of the resonant converter is disclosed in the following references: U.S. Pat. No. 3,818,312 issued on June 18, 1974, an article entitled "Voltage-Mode Resonant Push-Pull Converter with Magnetic Power Controllers" by T. Ninomiya et al in the 18th Annual IEEE Power Electronics Specialists Conference (PESC), June 23, 1987, pp. 339-347; an article entitled "Characteristics of Parallel Resonant Push-Pull Converter Controlled by Magnetic Amplifiers" by T. Ninomiya et al in the Journal of Japan Society of Applied Magnetics, vol. 11, No. 2, 1987, pp. 379-382, and an article in "Transister Technique" published by CQ Publishing Co., Ltd., 1987, No. 3 pp. 406-407, which are incorporated herein by reference.
On the other hand, a switching system is generally used for converter circuits for obtaining stabilized D.C. voltages from the commercial power supply or inverter circuits for providing a stablized D.C. source or high frequency A.C. source. However, the prior art switching system suffers from a disadvantage that when the output current of the converter is increased, the oscillation operation may be stopped. The cause of this has not been clarified. Further, the prior art switching system is difficult to miniaturize at a higher switching frequency due to the switching loss and switching noise which occur from increased superposition of the current and voltage due to leak inductance between the primary and secondary wirings when the switching devices turn on or off. On the other hand, it is desired to miniaturize a resonant power source at a high switching frequency; the resonant power source is provided with an LC resonance circuit in its main circuit to provide sinusoidal current and voltage waveforms and to decrease the superposition period of the voltage and current thereby reducing the switching loss and the switching noise harmonic component.
An example of such a resonant power source is a resonant inverter having a push-pull arrangement in its main circuit, such as disclosed in JP-A-48-8429. This resonant power source, however, the following disadvantage. In such a resonant power source, in which a resonant reactor is connected with a D.C. power source obtained from the commercial power supply, at least one of two switching devices used must be always placed in its conducting state. However, both switching devices are simultaneously turned off when the oscillation is compulsorily and urgently stopped through the remote on-off control from outside and the internal protection circuit. Then, a large voltage surge is produced due to the magnetic energy stored in the resonant reactor. This voltage surge is applied to the main switching devices which may be destroyed. Thus, the above resonant power source can not satisfactorily perform the above control and so has been applied to its limited use.
Further, the prior art resonant power source has a disadvantage that when, the voltage induced in the wirings for self-excited oscillation in the converter which is prepared to initiate the oscillation does not reach the voltage which enables the switching devices to be conductive, the oscillation can not be initiated.